Sliding dovetails provide several advantages over a dado joint. First, they're stronger because they don't rely solely on glue. Second, the shoulders of the dovetailed piece hide the edges of the slot, much as a tenoned workpiece hides a mortise. And the exposed dovetail provides a visible signature of your craftsmanship.
To make this joint, you'll rout the dovetail slot first, then cut and size the dovetail to fit. For the slot you'll need a handheld router and the dado jig shown below. In addition to a 5⁄8 " guide bushing, you'll need a 1⁄4 " spiral downcut bit for clean cuts (a 1⁄4 " straight bit will also work) and a 1⁄2 " dovetail bit. We chose a 7° dovetail bit because it leaves a thicker, stronger "neck" on the dovetail than a 14° bit would. You will shape the dovetail on a router table.
For testing the router-table setup, prepare a panel the same thickness and about the same width as the project panels to be dovetailed.
Lay out; then rout
You will cut the dovetail slot in two operations. First, to reduce the load on the dovetail bit, remove most of the waste with the spiral downcut bit. Then switch to the dovetail bit to complete shaping the slot. Because the dovetail slot tapers from bottom to top, the inside edges of the jig do not align with the top edges of the slot, photo below. To reposition the jig accurately for each operation, use the jig to draw layout lines on your workpieces to define both sides of the slots.
Install the bushing and spiral downcut bit, and set the bit's cutting depth to 1⁄16 " less than the final depth of the slot. Clamp the jig along the layout lines and rout across the panel, photo below. Rout each slot with this setup.
Quick Tip! Make a setup gauge, too. Rout a slot across a piece of scrap to make a gauge for setting up the router table later.
Mount the dovetail bit in the router and set the final depth. For the bookcase, the slot depth matches the rabbet for the back, photo below.
Reposition the jig over the first slot, aligning it carefully with the layout lines, and clamp it down. Rout along both edges with the dovetail bit. Repeat this process for the remaining slots, and make a pass on your scrapwood setup gauge to complete it.
Now to the router table
Secure the dovetail bit in your table-mounted router and set the bit height using the scrapwood gauge, below.
To steady the panels, attach a tall auxiliary fence to your router-table fence and position it to expose about one-third of the bit's width. Make cuts on your test panel with each face against the fence, photo below.
Test the fit of the dovetail in a slot, and adjust the fence as needed to achieve a fit that slides in with just a little hand pressure or light taps from a mallet. Refer to (Trouble)shoot the gap, below, for help fine-tuning the bit height.
Quick Tip! If a dovetail doesn't fit well in the slot, just cut off the end of the test panel and try again after making adjustments to the fence and/or bit.
Once the test joint fits well, rout a dovetail on each end of the top and bottom panels.
Slide it all together
Get a feel for the assembly process by dry-fitting the parts first. For the bookcase, assemble the top and bottom with one side, then slide the remaining side in place.
Quick Tip! To fine-tune a dovetail's thickness, cut a section of the dovetail from your test piece, wrap it with sandpaper, and use it as a sanding block.
If everything fits well, disassemble the parts.
Because a well-cut sliding dovetail has a strong mechanical connection, glue simply keeps the dovetailed piece from moving within the slot. We use white glue because, being thinner than yellow glue, it serves as a lubricant, helping the pieces move against each other. And, its longer open time allows more time for assembly.
Apply glue to the leading edges of the slots in one side panel, then begin reassembly. Repeat for the slots in the remaining side and slide it in place.